Turns out Fios was medium bitrate 1080i, Amazon’s HBO is high bitrate 1080p.

Yep, that's the norm. Amazon also has a good amount of stuff in 4K.

and they're good about the experience. on two separate occasions, i've had movies streaming through Amazon buffer for about 20 seconds total. and both times they sent me refunds for my rentals without me even contacting them bc it didn't even bother me that much.

i'm sure the refunds happen when the buffering is due to their servers and not your ISP, but still, not bad.